This invention relates generally to electrolytic cells and more particularly to the method of making wire reference electrodes to monitor voltage levels within the cell.
Typically the voltages of electrodes in either diaphragm type of chloralkali electrolytic cells or the more recently developed filter press membrane type of chloralkali electrolytic cells have been measured by use of a Luggin capillary tube that is positioned adjacent the electrode and which passes through the cell housing or electrode frame to a reference electrode placed outside of the cell. Such a Luggin capillary tube is inserted through the cell wall on top of the electrolytic cell by being passed through a polyethylene grommet or other appropriate seal and extended downwardly to a position adjacent the center of the electrode, for example, a cathode. The Luggin capillary tube is then connected by a salt bridge or liquid junction to a separate calomel reference electrode situated externally of the cell. This system of measuring electrode voltages does not permit the positioning of the reference electrode physically in the environment and at the exact location where the electrode potential to be measured exists. This method of measuring electric potential for electrodes is better suited for laboratory testing where electrode potentials must be measured.
The use of Luggin capillaries in electrolytic cells that generate gases creates further problems which are well known in the art. The Luggin capillary tube must have a continuous or unbroken stream of electrolyte in the tube throughout its length. One proven method of initially achieving this is by drawing the electrolyte through the tube by a syringe or other type of suction device in order to have sufficient electrolyte flow to obtain readings. However, gas generation creates bubbles that can block the relatively small capillary tube opening after the suctioning of electrolyte through the tube. This blockage, caused by the nucleation and growth of bubbles around the mouth of the tube, blocks the flow of electrolyte and causes a break to occur in the continuous stream of electrolyte along the tube's entire length. A similar obstruction can be created merely by the transfer or deposition of bubbles from the solution which were caused by a high level of agitation or rapid flow rate of the electrolyte fluid in the cell adjacent the electrode surfaces. Additionally, concentrated electrolyte solutions can salt out or freeze in the tube, thereby blocking liquid flow through the tube. To avoid this, once the salt bridge is established additional dilute electrolyte is normally fed into the tube.
It is also possible that the continuous stream of electrolyte which must be maintained through the Luggin capillary tube to the saturated calomel reference electrode is not identical to the electrolyte to which the electrode for which the potential is being recorded is exposed. This occurs when the dilute electrolyte solution flow is maintained downwardly through the capillary tube from an external reservoir into the cell electrolyte to avoid the gas bubble blockage at the tube's mouth or blockage within the tube from the aforementioned salting out of electrolyte. This flood of dilute solution does not permit an exact initial voltage reading to be obtained since the dilution of the electrolyte changes the measured voltage. In fact, this situation severely limits the utility of Luggin capillaries in conjunction with a saturated calomel reference electrode since they typically provide a potential recording only for that short window of time when solution is flowing through the tube and are not suitable for continuous or extended potential measurements.
Any of these conditions affect the accuracy of the reading obtained from the reference electrode using a Luggin capillary and, in fact, may obstruct the entire operation of the Luggin capillary.
Attempts to use Luggin capillaries in commercial electrolytic cells have proven them not to be suitable for commercial operations because of the practical problems encountered and their inherent limitations beyond those already enumerated. For example, the occurrence of an alternating current (AC) signal or ripple in the plant power supply will create rapid voltage changes which cannot be sensed by Luggin capillaries. Although these rapid voltage changes are not necessarily detrimental to the electrolysis, the potential in the Luggin capillaries cannot change rapidly enough and will, therefore, affect the reference electrode and its readings. Additionally, the length of the capillary tubes required for commercial sized cells could extend to twenty feet in length in order to connect to the external reference electrode. This length of tubing demands a very high internal pressure in order to keep the solution flowing and sweep any gas bubbles out of the tubing. If the necessary pressure to accomplish this is approached, the capillary tubes tend to leak from cracks or other failures or they pop off of their fittings. The latter event results in the spraying of hazardous caustic or other electrolyte about the cell plant building.
An obstruction problem can also result where salt bridges or liquid junctions are used with reference electrodes. These can become clogged, providing the same type of a problem encountered with gas bubble in the measurement of the potential and operation of the electrodes.
The desire to obtain electrical potential readings in the exact location where the potential to be measured exists by the insertion of reference electrodes into the cell has created additional problems. The harsh effect on the reference electrodes of the electrolytes encountered when the reference electrodes are inserted within the cell has been a persistent problem affecting the durability of the materials used to construct these electrodes. The corrosiveness of the anolyte and catholyte fluids tends to destroy the materials used. Reference electrodes with large diffuse Luggin openings also have been employed in attempts to avoid blockage problems. However, these electrodes have an electrical resistivity that is not uniform about their exposed surface. This non-uniform resistivity results in erroneous measurements since the voltage readings tend to be averages. This is especially true when the electrodes are subjected to high voltage gradients. Attempts to solve this problem have lead to the development of relatively costly structures either with a separate reference electrode or the incorporation of the reference electrode into existing electrodes. These devices utilize an annular element of porous material to close a cavity between the body portions of the reference and measuring electrodes to create an isolated cavity for the reservoir of electrolyte and a reference junction of uniform resistance over all radial segments. The ability to incorporate these types of structures in the commercial electrolytic cell has been difficult because of space requirements and the costs.
The development of wire reference electrodes has provided an approach that permits the electrode potentials to be monitored and recorded in commercial chloralkali electrolytic cells. However, prior wire reference electrodes have encountered the aforementioned durability problem, especially on the cathode side of the cell where the concentrated caustic solution tends to dissolve the wire. This is especially true in wire reference electrodes wherein a platinized platinum wire is employed. The dissolution because of the apparent high porosity of the exposed surface will occur over too short a period of time, often only several days, and limit the practical utility of these types of reference electrodes in commercially operating cells.
It has also been found that the seal around the wire separating the lead-in wire from the exposed reference wire portion in the wire reference electrodes is critical. It has been discovered that if electrolyte, especially the caustic solution, leaks backwardly between the exposed reference wire portion and the shielding that encases the lead-in wire, a second potential may be generated. This is particularly true at the weld point of the reference wire portion to the lead-in wire which is used so that the reference wire electrode may be connected over a substantially long distance to monitoring apparatus, such as volt meters, externally of the cell. Electrolyte solution wetting the weld joint will allow an electrochemical reaction, such as corrosion, so that there will be one potential at the wire reference electrode and another as a result of the reaction at the lead-in wire, despite the use of a polyfluorinated hydrocarbon insulating tube.
Where such reference wire electrodes have been utilized in the filter press membrane type of chloralkali electrolytic cells there is the potnetial for the electrodes to accidentally puncture the membranes, thereby reducing the efficiency of the cell operation. However, because of the utility of these wire reference electrodes to measure the total cell voltage, the anode-to-reference electrode, reference electrode-to-membrane-to-reference electrode, and the reference electrode-to-cathode voltages, continued efforts have now resulted in the solution of the aforementioned problems in the design of the present invention. The method of making the newly designed structure permits the use of the product reference electrodes to monitor electrode potentials over extended periods of time in commercial cells, as well as permitting fast transient studies of the operating cell conditions to be made where Luggin capillaries and external reference electrodes are not useful because of the high impedance level present that distorts the output voltage signal.